Search

In this paper, we have investigated the high temperature characteristics of monolithically integrated LED and MOS-channel HEMT in GaN. The monolithically integrated LED/MOSC-HEMT was implemented by using a selective epi removal approach. The temperature dependence of optical and electrical characteristics of the integrated LED is found to be comparable to that of discrete GaN based LEDs. On-resistance of the MOSC-HEMT shows gradual increase with temperature (~1.6× increase from 25 °C to 225 °C) whereas LED LOP shows rapid decrease with temperature (~6× decrease from 25 °C to 225 °C). Light output of the integrated LED is modulated by the MOSC-HEMT gate bias up to 225 °C.

The advent of a wide-bandgap GaN p-n junction has enabled highly efficient blue light-emitting diodes (LEDs) in GaInN/GaN heteroepitaxy. The system also enables a much wider range of emission wavelengths. We summarize progress in epitaxial materials development of green, yellow, and orange direct-emitting LEDs that bypass the steps of external phosphor conversion to achieve higher stability, efficiency, and higher overall color rendering quality for wider adoption of one of the widest power savings resources so far identified.

Low-resistance Ohmic contacts are essential for the fabrication of electrical devices. While low contact resistance has been achieved to p -type layers or n -type layers separately, contacts are likely to degrade when both types need to be integrated into a single fabrication process, in particular when prior mesa etching is required. We present a solution to the problem, resulting in low-resistance Ohmic contacts on n -type GaN layers without post-deposition thermal anneal, while maintaining the quality of typical p -type contacts. We implement an integrated process for both, n - and p -contacts, involving an oxygen pretreatment to fabricate light emitting diodes with lower series resistance in the contacts and lower voltage drop at high current when compared to separately optimized contacts.

We demonstrate photocurrent (PC) spectroscopy of GaInN/GaN multiple quantum well structures with electroluminescence peak wavelengths near 550 nm. Two classes of closely related samples were grown by metal-organic vapor phase epitaxy on c -plane (0001) sapphire substrate. Although nominally identical, one set appears optically transparent while the other appears black to the eye. The PC spectrum of the black sample shows a strong enhancement of absorption at wavelengths longer than 450 nm. In UV microscopy clusters of dark areas are found in the black sample. Such findings are commonly ascribed to areas of Indium phase segregation but require further study. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report the development of a novel ridge-type AlGaInN-based laser diode structure fabricated by a metal organic vapor phase epitaxial regrowth method. The selective-area regrowth of AlGaN was optimized and compared with regular planar growth. An AlN fraction of 13% in the regrown ridge was concluded from the cathodoluminescence spectrum. The laser diode structure showed superluminescence at 465 nm under optical excitation. Under electrical excitation a super-linear increase of the light output power was observed.

GaN, LED, nano-structure ** Corresponding author: e-mail starkc2@rpi.edu, Fax: +1 518 276 8042 ** e-mail wetzel@rpi.edu, Phone: +1 518 276 3755, Fax: +1 518 276 8042 The mesa width of green light-emitting diodes (LEDs) was scaled from the micrometer to the nanometer range. The devices were patterned by electron beam lithography and dry etching and contained up to 100 parallel nano-stripe LEDs connected to one common contact area. The mesa width was varied over 1 μm, 200 nm, and 50 nm. The LEDs were characterized electrically and optically, and the peak emission wavelength was found to depend on the lateral structure size. Light emission from the pat-terned region was found to be 4 times brighter than from the un-patterned regions.

The light output of 530 nm green GalnN/GaN light-emitting diodes on sapphire has been nearly doubled by the insertion of a 130-nm GalnN underlayer (UL) between the n-GaN electron injection layer and the quantum-well (QW) active region. Under variation of the alloy composition, best results were obtained for an x = 6.3% Ga1-xInxN UL. By low-temperature depth-resolved cathodoluminescence spectroscopy, an interplay of the impurity-related donor-acceptor pair recombination, the UL, and the QW emission has been observed. We propose that the resonance and level alignments between the defect and UL levels reroute excitation toward radiative recombination in the QWs.

We report the growth and structural characteristics of green and yellow (529-576 nm) GaInN/GaN light emitting diodes (LEDs) grown on two types of c-plane substrates - bulk GaN and sapphire. In this longer wavelength range, depending on the substrate, we find different strain relaxation mechanisms within the GaInN/GaN quantum well (QW) region. In optimized epitaxy, structures on sapphire that contain a low density of threading dislocations (TDs) within the n-GaN show virtually no generation of additional misfit dislocations (MDs) (

The junction temperature of homoepitaxial green and blue GaInN/GaN quantum well light emitting diode (LED) dies is analyzed by micro-Raman, photoluminescence, cathodoluminescence mapping, and forward-voltage methods and compared to finite element simulations. Dies on GaN substrate and sapphire were analyzed under variable drive current up to 200 mA (246 A/cm2). At 100 mA, dies on bulk GaN remain as cool as 355 K (83 °C) while dies on sapphire heat up to 477 K (204 °C). The efficiency droop and spectral line shift in green LEDs with increasing current density can now be separated into electrical and thermal contributions.

GaInN/GaN light emitting diodes are the primary choice for efficient green light emitters, yet their performance limiting factors have yet to be identified. Here we perform a low-temperature luminescence study in depth-resolved cathodoluminescence. A series of LEDs with a combination of blue and green light emitting quantum wells exhibiting low and high electroluminescence output power were compared. In all samples, a band of donor-acceptor pair recombination was identified at 77 K. In the samples with lowest performance, such transitions located within the active region of the quantum wells had a particular strong contribution to the spectrum. In LEDs of higher performance, such luminescence was substantially suppressed within those layers. We argue that the higher performance of LEDs without such donor acceptor transition bands may be associated with the absence of corresponding dopant impurities and indicate a higher epitaxial perfection within the active quantum well region.

An enhancement of radiative recombination in GaInN/GaN heterostructures is being pursued by a reduction of defects associated with threading dislocations and a structural control of piezoelectric polarization in the active light-emitting regions. First, in conventional heteroepitaxy on sapphire substrate along the polar c-axis of GaN, green and deep green emitting light-emitting diode (LED) wafers are being developed. By means of photoluminescence at variable low temperature and excitation density, internal quantum efficiencies of 0.18 for LEDs emitting at 530 nm and 0.08 for those emitting at 555 nm are determined. Those values hold for the high current density of 50 A/cm 2 of high-power LED lamps. In bare epi dies, we obtain efficacies of 16 lm/W. At 780 A/cm 2 we obtain 22 lm when measured through the substrate only. The 555 nm LED epi material under pulsed photoexcitation shows stimulated emission up to a wavelength of 485 nm. This strong blue shift of the emission wavelength can be avoided in homoepitaxial multiple quantum well (MQW) and LED structures grown along the non-polar a- and m-axes of low-dislocation-density bulk GaN. Here, wavelength-stable emission is obtained at 500 and 488 nm, respectively, independent on excitation power density opening perspectives for visible laser diodes.

We demonstrate homoepitaxial growth of GaInN/GaN-based green (500–560 nm) light emitting diodes (LEDs) on a-plane and m-plane quasi-bulk GaN prepared by hydride vapor phase epitaxy (HVPE). We find that in order to achieve an emission peak wavelength beyond 500 nm, a minimum InN-fraction of ∼14% is needed for both, a- and m-plane quantum wells (QWs), while ∼8% are enough for c-plane-oriented QWs. Besides increasing the InN-fraction in these non-polar QWs, widening the QW also proves to effectively shift the emission to longer wavelengths without loosing efficiency with the benefit of maintaining a low InN-fraction.

GaInN/GaN multiple quantum well light-emitting diode structures in polar c -axis and non-polar m -axis growth have been compared in terms of luminescence properties. Grown under identical conditions, under low excitation density the c -axis structure has a luminescence maximum at 558 nm while the m -axis structure shows a maximum at 488 nm and shows superluminescence at 485 nm under high photoexcitation density. Under the same conditions, on increasing the excitation power, the peak intensity increases 40 fold in the m -axis structure without any variation of the emission wavelength. In similar but separately grown c -axis structures without a p-side, luminescence shifts from 555 nm at low excitation density to superluminescence at 485 nm under high excitation. The coincidence, of the superluminescence wavelength in the polar structure with the stable peak wavelength in the non-polar one, suggests that the wavelength shift in the polar structure is due to its piezoelectric polarization. The absence of such effects in the m -axis-grown structure therefore suggests a stronger dipole matrix element, potentially enabling higher quantum efficiencies and suitability for high efficiency light-emitting diode and laser diode designs in the green spectral region.

The efficiency of green GaInN/GaN light emitting diodes (LEDs) is known to drop under high current densities. To explore the reason, 535 nm emitting GaInN/GaN multiple quantum well (MQW) and GaN epilayers were characterized using Z-scan techniques under comparable excitation conditions with a continuous wave laser. Two wavelengths, 514 nm and 488 nm, were selected right below and above the apparent optical absorption edge. At 514 nm, a very large nonlinear absorption coefficient β = 2.6 cm/W was obtained. This leads to a 20% absorption enhancement at a photon flux of 21 kW/cm2. We attribute this to nonlinear free-carrier absorption. On the other hand, at 488 nm, a nonlinear saturable absorption with β = –1.7 cm/W was observed. This induced transparency indicates photon bleaching in the MQW. Apparently, free carrier dynamics strongly affects optical nonlinearity, while nonlinear absorption provides a small contribution to the limitations of current green LEDs. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We analyze GaInN based light emitting diodes emitting in the range of 400-550nm using a new intensity-quantitative spectroscopic cathodoluminescence mapping method. Spectroscopic information of arbitrary sample locations is generated from sequences of quantitative image scans. From the temperature dependence of the intensity, we derive thermal activation energies of the dominant loss processes. Those compare well with the hole binding energies in the piezoelectric and quantized quantum well structures.

Energy efficient lighting can be achieved by help of high efficiency light emitting diodes (LEDs). Substantial performance gains are feasible in the green – 500–555 nm – spectral region by implementing proper design of the active region. Here we analyze external quantum efficiency of high performing LED dies as a function of current and temperature in order to formulate relevant optimization rules. Among a large sample set we identify traits that clearly correlate with respective device performance. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We demonstrate homoepitaxial growth of GaInN/GaN-based light emitting diodes (LED) on quasi-bulk GaN with an atomically flat polished surface. The threading dislocation densities of the epitaxial layers were 2-5 x 10 8 cm -2 which was one order of magnitude less than those grown on c-plane sapphire substrate. The growth defects introduced during the epitaxial process were also one order of magnitude smaller than those grown on the sapphire substrate. The crystalline quality and the optical properties of the epitaxial layer and device performance were much improved. The optical output power of the light emitting diode increased by more than one order of magnitude compared to those on sapphire substrate.

A charge profiling of GaInN/GaN hetero-polarization structures typical for high power 525 nm green light emitting diodes is presented. We identify three individual charge maxima of some 1.17 x 10 12 cm -2 electrons with a line width as narrow as 17 nm corresponding to three individual but widely separated quantum wells. The sheet charge density indicates negligible screening of the polarization charges. The voltage required to deplete each well is found to be ΔU = -0.4 ± 0.1 V and approximates the voltage drop across the polarization dipole of the quantum well. The modulation frequency dependence reveals a characteristic time constant of 1 μs for the dominant trapping and ionization process. Such analysis of high performance green light emitters proves a suitable tool for further device optimization.

Progress of light emission efficiency in the green and deep green spectral range is crucial for the success of all solid state lighting. Yet, power performance in GaInN/GaN quantum well light emitting diodes (LEDs) in the green still lag far behind the development of red and blue. A key signature is a steep roll-off in emission power when the wavelength is increased from 460 nm in the blue to 525 nm in the green and 550 nm in the deep green. The physical reason needs to be identified to overcome such limitations. A meaningful study requires a large base of sample data to meliorate the effects of sample variations. We analyze the data of hundreds of growth runs for several distinct epi development stages. In particular, when comparing data for two different epi optimization concepts we find a strong variation of their respective roll-off slopes in the green. These findings show that the roll-off slopes are not universal and can be controlled at a high performance level by advanced epi optimization. We find that an optimization of the quantum well smoothness leads to much weaker roll-off behavior and boosts performance of newly optimized green LED dies (2.4 mW at 20 mA, unpackaged, 527 nm dominant). Our results should open pathways on how to further extend power performance of deep green LEDs. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We studied the optical properties of metalorganic chemical vapour deposited (MOCVD) InGaN/GaN multiple quantum wells (MQW) subjected to nitrogen (N) implantation and post-growth annealing treatments. The optical characterization was carried out by means of temperature and excitation density-dependent steady state photoluminescence (PL) spectroscopy, supplemented by room temperatura PL excitation (PLE) and PL lifetime (PLL) measurements. The as-grown and as-implanted samples were found to exhibit a single green emission band attributed to localized excitons in the QW, although the N implantation leads to a strong reduction of the PL intensity. The green band was found to be surprisingly stable on annealing up to 14006C. A broad blue band dominates the low temperature PL after termal annealing in both samples. This band is more intense for the implanted sample, suggesting that defects generated by N implantation, likely related to the diffusion/segregation of indium (In), have been optically activated by the thermal treatment, The authors acknowledge FCT for the final funding from PEst-C/CTM/LA0025/2013-14, PTDC/CTM-NAN/2156/2012, PTDC/FIS-NAN/0973/2012 and RECI/FIS-NAN/0183/ 2012 (FCOMP-01-0124-FEDER-027494) projects. J. Rodrigues thanks FCT for her PhD grant, SFRH/BD/76300/2011. ARC acknowledges financial support under the ‘Juan de la Cierva’ program (MECO, Spain) through grant JCI-2012-14509.

We model and optimize various light emitting diode structures under bias voltage to maximize emission efficiency with particular respect to piezoelectric polarization. We compare polar and non-polar structures, namely, wurtzite c-plane, a-plane, (11–22) semi-polar, and (001) cubic crystal orientations in self-consistent Schrodinger-Poisson and drift-diffusion models. We consider both structures strained to a GaN pn-junction and strain-reduced systems based on GaInN templates. In light of numerous experimental findings of the actual electric field strength, we find it necessary to reduce the piezoelectric coefficients over those commonly cited. A weaker variation with composition or wavelength is the consequence. For the non-polar and cubic systems, we find a 22% increase of the electron-hole overlap and an 18% increase for the c-plane strain-reduced system at an InN fraction of x = 0.30 when compared to standard c-plane structures. For the green and longer wavelength range, we find that strain-reduced and...

Low-defect density non-polar a-plane Ga1-yInyN layers on r-plane sapphire substrates are reported by implementing self-assembling nanopatterning in metalorganic vapor phase epitaxy. Nanopillar capping and void formation in regrowth lead to a 90% defect reduction. An ex-situ Ni layer transforms into a nanoisland etch mask to pattern GaN templates. a-Plane GaN and Ga1-yInyN layers with an InN content in the range of y = 0.04–0.11 are then regrown. Both exhibit a low density of basal-plane stacking faults of (4.6 ± 1.3) × 104 cm−1 by transmission electron microscopy analysis. Growth parameters and the template pattern are discussed by help of an X-ray rocking curve analysis. We find pattern the fill factor and V/III ratio to dominate the defect reduction. Resulting layers should enable efficient long-wavelength light-emitting and solar cell devices.

We report the development of non-polar light emitting diodes (LEDs) covering the emission spectra from 480 to 520 nm (dominant wavelength), i.e. from cyan to the green region. The devices are obtained via GaInN-based homoepitaxy on non-polar m -plane GaN bulk substrate. Owing to the absence of piezoelectric polarization, these LEDs exhibit stable emission color with a wavelength shift less than 3 nm when the drive current density is changed from 0.1–30 A/cm2. However, we observe a decreasing electroluminescence efficiency as the emission wavelengths increases from cyan to green. We tentatively attribute this to a higher density of defects in the longer wavelength structures. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

GaN-based MQWs-SCH laser diodes (LDs) with Fabry–Perot resonator mirrors fabricated by focused ion beam (FIB) etching were demonstrated for the first time. The diodes show lasing by pulsed current injection at room temperature with a lasing wavelength near 410 nm. FIB etching of the mirrors significantly reduced the threshold current from 1.25 to 0.75 A. In addition we studied the dependence of I-L characteristics on the successive rotation of the etched mirror of a single device and found a strong angular dependence. A similar study of the tilting angle revealed a very weak variation.

Under external laser excitation, we studied the electroluminescence (EL) of GaInN/GaN green light emitting diodes (LEDs) grown on sapphire by metal organic vapor phase epitaxy. The EL intensity was found to be greatly enhanced under the photon bias. This enhancement strongly supersedes the photoluminescence (PL) signal. The EL enhancement varied with injection carrier density. The relative EL enhancement under 325-nm laser bias starts with a high ratio and decreases monotonically. This increase of the EL intensity is tentatively attributed to a balance between photocarrier screening and carrier drift within the active region. These findings elucidate the transition from highly efficient recombination at low current density to the region of efficiency droop at high current densities. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We investigate pn-junction heating in 525 nm green GaInN/GaN quantum well light emitting diode dies by micro-Raman and photoluminescence spectroscopy. We compare dies grown on GaN and sapphire substrates of different sizes at current densities up to 267 A/cm2. Analysis of the experiments was supported by realistic three-dimensional finite-element simulations of the thermal properties. Sapphire based LED dies reach a junction temperature as high as 240 °C at a current density of 133 A/cm2, while those on bulk GaN only heat to 83 °C. This corresponds to a thermal resistance of 428 K/W and 63 K/W, respectively. By help of this calibration, the role of junction temperature in the spectral light output performance is identified. From the numerical simulation, dimensions and thermal conductivity of the substrate are identified as the dominant control parameters. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The strong correlation between V-defect density in the active region and light output power of GaInN based green and deep green light emitting diodes (LEDs) has been evaluated. The formation of such defects is confirmed as the major cause for rough growth morphology, drift of well and barrier widths, emission linewidth broadening, large color shift with current density, and thus poor light emission performance. We have optimized the epitaxial processes for the active region and successfully eliminated such defects from the entire device structure. The electroluminescence intensities from these V-defect-free devices with dominant wavelengths ranging from 520 to 570 nm, improve by a factor of two under high injection current density up to 100 A/cm2. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Development progress of high power 525 nm green LEDs employing GaInN/GaN active layers optimized for high morphological homogeneity is presented. We analyze the benefits of this process for the reproducibility in epitaxial growth and processing as well as across the wafer uniformity. We show that by means of our approach performance in terms of dominant wavelength and output power are highly reproducible across the wafer and from run to run. This lends special relevance also to any spectroscopic analysis of such sample material.

Efficiency droop and green gap are terms that summarize performance limitations in GaInN / GaN high brightness light emitting diodes (LEDs). Here we summarize progress in the development of green LEDs and report on time resolved luminescence data of polar c -plane and non-polar m -plane material. We find that by rigorous reduction of structural defects in homoepitaxy on bulk GaN and V -defect suppression, higher efficiency at longer wavelengths becomes possible. We observe that the presence of donor acceptor pair recombination within the active region correlates with lower device performance. To evaluate the aspects of piezoelectric polarization we compare LED structures grown along polar and non-polar crystallographic axes. In contrast to the polar material we find single exponential luminescence decay and emission wavelengths that remain stable irrespective of the excitation density. Those findings render high prospects for overcoming green gap and droop in non-polar homoepitaxial growth.

Amazing performance in GaInN/GaN based LEDs has become possible by advanced epitaxial growth on a wide variety of substrates over the last decade. An immediate push towards product development and worldwide competition for market share have effectively reduced production cost and generated substantial primary energy savings on a worldwide scale. At all times of the development, this economic pressure forced very fundamental decisions that would shape huge industrial investment. One of those major aspects is the choice of epitaxial growth substrate. The natural questions are to what extend a decision for a certain substrate will limit the ultimate performance and to what extent, the choice of a currently more expensive substrate such as native GaN could overcome any of the remaining performance limitations. Therefore, this project has set out to explore what performance characteristic could be achieved under the utilization of bulk GaN substrate. Our work was guided by the hypotheses that line defects such as threading dislocations in the active region should be avoided and the huge piezoelectric polarization needs to be attenuated – if not turned off – for higher performing LEDs, particularly in the longer wavelength green and deep green portions of the visible spectrum. At theirmore » relatively lower performance level, deep green LEDs are a stronger indicator of relative performance improvements and seem particular sensitive to the challenges at hand.« less

Two different approaches to optimize the growth conditions for high-power green light-emitting diodes (LEDs) using Ga1−xInxN∕GaN metalorganic vapor phase epitaxy are discussed. We compare typical results in terms of morphology, photo-, and electroluminescence properties. We find good results for an optimization of the lateral morphological homogeneity of the active region. An extension of growth conditions for the active layers of blue LEDs was misleading. This suggests that different emission processes are involved in blue and green LEDs. We achieve die performances of 2.5mW at 523nm (526nm dominant) for low forward voltages of 3.4V at a typical drive current of 20mA.

In search of a better transparent contact to p-GaN, we analyze various metal/indium-tin-oxide (ITO) (Ag/ITO, AgCu/ITO, Ni/ITO, and NiZn/ITO) contact schemes and compare to Ni/Au, NiZn/Ag, and ITO. The metal layer boosts conductivity while the ITO thickness can be adjusted to constructive transmission interference on GaN that exceeds extraction from bare GaN. We find a best compromise for an Ag/ITO (3 nm/67 nm) ohmic contact with a relative transmittance of 97% of the bare GaN near 530 nm and a specific contact resistance of 0.03 Ω·cm2. The contact proves suitable for green light-emitting diodes in epi-up geometry.

The n-GaN layer of c-plane GaInN/GaN light emitting diodes (LEDs) on sapphire was modified to contain a pattern of SiO 2 nanorods. This embedded pattern of 300 nm long rods and diameter of 200–400 nm was created by thermal agglomeration of a Ni mask layer and subsequent dry-etching. The light output power (LOP) and external quantum efficiency (EQE) of the resulting LEDs increased both by some 25% to 40% depending on current density over the unpatterned structure. The increase is thought to be primarily the result of enhanced light extraction efficiency induced by enhanced scattering at the substrate-side interface.

Nitride-based light emitting diodes (LEDs) have received great attention with emission wavelength from ultraviolet to green. However, conventional c-plane GaN LEDs suffer from polarization-related electric fields, which result in poor carrier recombination efficiency and red-shift in the emission wavelength. In order to reduce the undesirable polarization effects, non-polar plane GaN LEDs have been proposed and reported. But the non-polar films also suffer from poor crystalline quality and rough surface morphology, especially for hetero-epitaxy. Several research groups have reported high performance non-polar GaN-based LEDs grown on bulk GaN substrates [1]. However, high price, small size, and non-uniformity in the substrates also make them difficult for mass production. On the other hand, nano-patterned a-plane GaN on sapphire substrate has been proposed as a solution. It is thought of as a method to achieve low cost, large area, and high uniformity a-GaN on sapphire. In this paper, self-assembled Ni nano-islands [2] are applied to form a-plane GaN nanorod template. We also grew blue and green [3] nano-patterned a-plane GaN LEDs on those nanorod templates.

Cubic GaInN/GaN multi-quantum wells (MQWs) are grown on V-grooved Si {100} substrates. The crystal phase is confirmed by electron-back scatter diffraction and cathodoluminescence shows room temperature MQW peak emission at 498 nm.

Energy efficiency has been the primary driving force for solid state lighting to replace wasteful incandescent lamps by light emitting diodes (LEDs). Recently, rising cost for rare earth metals has redoubled the push to also replace fluorescent and compact fluorescent lighting. Phosphors in fluorescent lamps heavily rely on rare earth metals and even first generation LEDs use such phosphors, albeit at much lower quantities. The role of phosphors to expand a narrow wavelength source into a wider spectrum is a very lossy process in itself and can be circumvented altogether by second generation LEDs, where the full visible solar spectrum is directly replicated by direct emitting LEDs. We here report progress of our work towards this goal, in particular by the development of high brightness direct emitting green group- III nitride LEDs.

Solid state lighting seeks to replace both, incandescent and fluorescent lighting by energy efficient light-emitting diodes (LEDs). Just like compact fluorescent tubes, current white LEDs employ costly rare earth-based phosphors, a drawback we propose to overcome with direct emitting LEDs of all colors. We show the benefits of homoepitaxial LEDs on bulk GaN substrate for wavelength-stable green spectrum LEDs. By use of non-polar growth orientation we avoid big color shifts with drive current and demonstrate polarized light emitters that prove ideal for pairing with liquid crystal display modulators in back light units of television monitors. We further offer a comparison of the prospects of non-polar a- and m-plane growth over conventional c-plane growth.

In absence of piezoelectric polarization along the growth axis, a- and m-plane green GaInN light emitting diodes manifest stable emission wavelength -- independent of the injection current density. The shift of the dominant wavelength is less than 8 nm when varying the forward current density from 0.1 to 38 A/cm2. Furthermore, the light emitted from the growth surface of such non-polar structures shows a very degree of linear polarization. This is attributed to a strong valance band splitting in such anisotropically strained wurtzite GaInN quantum wells . Such light emitting diodes show a high potential for energy efficient display applications.

Received 2 September 2009; revised manuscript received 29 January 2010; published 22 March 2010 The creation of symmetrical pairs of inclined dislocations was observed in the GaInN/GaN quantum wells QWs of c-axis grown green light-emitting diodes LEDs on low-defect density bulk GaN substrate, but not in green LEDs on sapphire substrate with high threading dislocation TD density. Pairs of dislocations start within 20 nm of the same QW and incline 18° – 23° toward two opposite 11 ¯ 00 directions or in a 120° pattern. We propose that in the absence of TDs, partial strain relaxation of the QWs drives the defect formation by removal of lattice points between the two dislocation cores. In spite of those inclined dislocation pairs, the light output power of such green LEDs on GaN is about 25% higher than in LEDs of similar wavelength on sapphire.

This work's objective was the development of processes to double or triple the light output power from green and deep green (525 - 555 nm) AlGaInN light emitting diode (LED) dies within 3 years in reference to the Lumileds Luxeon II. The project paid particular effort to all aspects of the internal generation efficiency of light. LEDs in this spectral region show the highest potential for significant performance boosts and enable the realization of phosphor-free white LEDs comprised by red-green-blue LED modules. Such modules will perform at and outperform the efficacy target projections for white-light LED systems in the Department of Energy's accelerated roadmap of the SSL initiative.

The green spectral region provides a formidable challenge for energy efficient light emitting diodes. In metal organic vapor phase epitaxy we developed GaInN/GaN quantum well material suitable for 500 - 580 nm LEDs by rigorous defect reduction and thrive for alloy uniformity. We achieve best results in homoepitaxy on polar c-plane, and non-polar a -plane and m -plane bulk GaN. By the choice of crystal orientation, the dipole of piezoelectric polarization in the quantum wells can be optimized for highest diode efficiency. We report progress towards the goal of reduced efficiency droop at longer wavelengths.

We have performed the epitaxial growth of GaInN based deep green (540–570 nm) light emitting diodes (LEDs) by introducing processes to avoid the formation of V-defects in the active region of the GaInN/GaN multiple quantum wells. For 541 and 559 nm LED wafers, the light output power (LOP) at 90 A/cm2 reaches as high as 14 and 8 mW, respectively. These values are 4 times larger than those of typical deep green LED wafers with average V-defect density of 2–5 × 108cm–2. Under pulsed operation (pulse width of 50 μs, duty cycle 1%), the external quantum efficiency exhibits a maximum of 39% at 16 A/cm2 in 558 nm LED die of size (350 μm)2. This type of V-defect-free LED die also shows excellent lifetime behaviour. In 168 hr stress test of 30 mA at 80 °C, LOP declines less than 5%. Apparently, eliminating V-defects from the active region significantly enhances the reliability of deep green LEDs. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the low-frequency current and light noise in 515 nm green GaInN/GaN quantum well LEDs. The current noise was the superposition of the 1/f and the generation-recombination (GR) noise. The characteristic time of the GR process was found to be proportional to the reciprocal current for the entire current range. This dependence is the characteristic for the monomolecular non-radiative recombination .The dominance of the nonradiative recombination is in agreement with the measured low external quantum efficiency (EQE)

Optical properties of green emission Ga0.80In0.20N/GaN multi-quantum well and light emitting diode have been investigated by using photoluminescence, cathodoluminescence, electroluminescence, and photoconductivity. The temperature dependent photoluminescence and cathodoluminescence studies show three emission bands including GaInN/GaN quantum well emission centered at 2.38 eV (~ 520 nm). The activation energy of the non-radiative recombination centers was found to be ~ 60 meV. The comparison of photoconductivity with luminescence spectroscopy revealed that optical properties of quantum well layers are strongly affected by the quantum-confined Stark effect.

Electroluminescence of GaInN/GaN multiple-quantum-well (QW) light-emitting diodes emitting in the green spectral region is analyzed at variable low temperature. Spectra are dominated by QW emission at RT throughout 7.7 K. Below 150 K, donor-acceptor pair recombination appears that must be assigned to residual impurities in either the barriers or the p-layers. The current-voltage behavior reveals shunt paths that carry up to several mA at low bias voltages. Below 20 K those paths are frozen out, but the device still emits predominantly from the QW. The peak energy exhibits a blue shift from RT to 158 K followed by a red shift from 158 K to 7.7 K. The deeper binding energy at low temperatures can hardly be affect by the injection current indicating that saturation of low-density states cannot be responsible for the transition between red and blue shifts.

Green and deep-green light emitting diodes are still the weakest link in energy efficient solid-state lighting. We analyze the limiting factors of the external quantum efficiency and summarize our approach of dislocation and polarization control.

Achieving p-type conduction in GaN has poised the material a power semiconductor on all fronts. Record conversion efficiencies and power densities in LEDs render it a platform of GW installation base. After commercial success as a blue light emitter, the emphasis now rests on the expansion of its capacity as a full spectrum direct emitter --- and absorber. Green and amber LEDs will allow full control of lighting quality and specificity in photochemistry. In early results we find evidence of temperature performance crucially superior to those of the incumbent. Next, we explore the same GaInN/GaN quantum wells as a concentration solar cell converter. While definitely limited to the short wavelength range so far, we find respectably high spectral conversion efficiencies and outstanding temperature stability. Such performance, however, is just early indication of what else lies ahead in terms of power electronics in the form of lateral and vertical transistor. In early results we monolithically integrate LED and transistor.

Increasing emission power and efficiency in green light emitting diodes is one of the big challenges towards all-solidstate lighting. The prime challenge lies in the combination of extension of wavelength from 470 nm blue to 525 nm green while maintaining the emission power level. Commonly a steep decrease in power is observed. In a broad development effort we have been able to ameliorate that decrease significantly and obtain bare die performance at 525 nm of 1.6 mW at 20 mA for 350x350 µm 2 dies. Here we discuss critical die performance and wafer yield aspects of our optimization approach to the active layer of the GaInN/GaN quantum wells.